The invention relates to a novel class of cinnamamides, a process for their manufacture and their use as ultraviolet (UV) stabilizers. Ultraviolet light (wavelengths between 280 nm and 400 nm) is known to degrade exposed organic matter. Materials used in exterior applications as well as interior applications (where there is exposure to UV rays through glass) such as fibers and fabrics for use in applications including marine sails and ropes, awnings, tents, flags, upholstery (including interior automotive fabrics), carpet, sports equipment, soft-sided luggage, seatbelt webbing, animal control webbing and clothing!, molded plastic parts for use in applications including automobile parts (such as mirror housings, door handles, body panels and bumpers), as well as, sports equipment and tool housings!, plastic films for applications including agricultural applications (such as greenhouse coverings, crop protection and food packaging), and packaging for chemical products (such as pesticides and fertilizers)!, and plastic coatings for applications including paint! are susceptible to photochemical degradation.
Specifically, polymers develop undesirable color or haze and subsequently lose their transparency and physical properties such as tensile strength, flexibility, gloss and impact resistance. Dyed, pigmented and mineral and glass filled plastics and textiles (e.g., carpet fibers) and color containing plastics e.g. aliphatic and aromatic polyamides including nylon 6 and nylon 6,6; polycarbonate; polyesters such as poly(ethyleneterephthalate) (PET) and poly(butyleneterephthalate) (PBT); polyolefins such as poly(ethylene), poly(propylene); aramids; acrylates; acetates; polyvinylchloride; polyimides; fluoropolymers; polyurethanes; polyacetals; polysulfones and polyaryletherketones! fade, become brittle, lose their elasticity and eventually completely deteriorate.
As a result, the plastics industry has developed a broad range of stabilizers to prevent UV degradation. These include, radical scavengers such as hindered amine light stabilizers (HALS), phosphites and phenolic antioxidants (AOs), thioethers, metal dithiolates, sulfoxides, among others and ultraviolet light absorbers such as benzotriazoles (BZTs), hydroxybenzophenones (HBPs), cinnamates, benzylidene malonates and nickel chelates. A broad review of these and other stabilizers can be found in J. F. Rabek, "Photostabilization of Polymers; Principles and Applications", Elsevier Applied Science, NY, 1990.
In addition to protecting against the damaging effects of UV light in the plastic itself, UV absorbing stabilizers can be used in transparent packaging to absorb UV light and thereby prevent damage to the package contents (such as food where light induced oxidation can occur in natural oils leading to undesirable odors and rancidity or drugs).
In order to be useful in plastics applications, a candidate UV light absorber must have a strong UV light absorptivity at wavelengths between 280 and 400 nm, be colorless, photostable, nonvolatile under processing and end use conditions, chemically inert during storage, processing and application conditions, and have an efficient mechanism for the conversion of light energy to heat without initiating radical or other degradation processes. Low volatility, chemical inertness, and thermal stability are particularly important considerations for the "engineering plastics" such as polyesters, polyamides, polycarbonates, polyacetals, polysulfones, polyimides, and polyaryletherketones where melt processing conditions often reach or exceed temperatures of 300.degree. C. with lengthy melt residence times. Few of the commercially available UV stabilizers can withstand these processing conditions for reasons discussed below.
The two dominant classes of commercially available UV absorbers are benzotriazoles (BZTs) and hydroxybenzophenones (HBPs). Both utilize an excited state intramolecular proton transfer from a hydroxyl group to dissipate light energy following absorption of ultraviolet light. Neither is without problems. The requisite hydroxyl groups of both the BZTs and the HBPs often result in severe color formation, especially in pigmented and filled plastics such as glass and mineral filled polyamides or polyesters. Often the color formation is a result of complexation with pigments, colorants, catalysts or fillers. In basic media, deprotonation of the phenolic hydroxyl groups of these compounds results in formation of strong yellow colors. Additionally, in hydrogen bonding plastics such as polyamides, the hydroxyl group is tied up via hydrogen bonding with the polymeric substrate which decreases the absorption strength of the stabilizer and narrows the absorption band, thus reducing the effective wavelength for the stabilizer and eliminating the mechanism for excited state energy dissipation.
In addition, many of the commercially available benzotriazoles are too expensive (at the typical required loadings in plastics) and/or volatile, and fume during processing. The lower volatility, higher molecular weight BZT dimers are costly and those that are connected via ester linkages can cause loss of molecular weight during melt processing of polyesters as a result of ester--ester interchange reactions.
For applications in which plastics are exposed to wavelengths below 320 nm, the BZTs are less effective because their absorption strength drops off below 315 nm, a critical absorption region for undyed polyester and polycarbonate stabilization. HBPs which absorb strongly in the general region of 340-360 nm also have a lower absorption strength in the general region of 290 to 330 nm. Many commercial HBPs are also too volatile for processing in many high temperature engineering resins and cause yellowing in polyesters. Finally, many commercial HBPs cause chain scission thereby degrading the molecular weight of polyesters such as PET and PBT during processing (e.g. spinning).
Two other general classes of absorbers are sold commercially. These are the oxanilides and cinnamates (esters of cinnamic acid). The oxanilides are thought to dissipate UV light energy by an intramolecular excited state proton transfer from an amide nitrogen to an amide carbonyl. These absorbers therefore can potentially be rendered ineffective by hydrogen bonding with the polymer resin in e.g. polyamides. The absorption strength of oxanilides is also somewhat low compared to other UV absorbers. The cinnamates (used as sunscreens) do not suffer from these same drawbacks since they have high absorption strengths and dissipate excited state energy through cis-trans isomerization of the cinnamate double bond (and therefore cannot readily be rendered ineffective by hydrogen bonding polymers). However, these materials are generally either too volatile to withstand processing at 250-315.degree. C. or they are oily liquids which cause lubrication in processing equipment (e.g. screw lubrication in extruders) and phase separation.
Ester--ester interchange is a problem for the monomeric cinnamates and prevents them from being used in melt-processed polyester objects such as fibers since the molecular weight of the polyester polymer is drastically reduced. In melt extruded polyamides, the basic cinnamate chromophore is destroyed, by a mechanism that most likely involves nucleophilic attack at the .beta.-position of the unsaturated cinnamate ester. It is also well known that cinnamates can undergo various photochemically induced cyclization reactions of which 2+2 photodimerization is a prime example. The dimerization destroys the chromophore necessary for UV stabilization.
Various cinnamamides have been used in the art as UV stabilizers for engineering resins. See U.S. Pat. Nos. 3,174,937 ('937 patent) and 3,272,855 ('855 patent) to General Aniline & Film Corporation and U.S. Pat. No. 4,883,653 ('653 patent) to Olin Corporation. The cinnamic acid amide dimers of the '653 patent have only hydrogen substitution in the .alpha. and .beta. positions. Thus, these molecules are susceptible to chromophore destruction via both nucleophilic conjugate addition and 2+2 photodimerization. The same is true of the cinnamic acid dimers disclosed in the '937 patent. The alkoxy substitution (RO) present in the compounds disclosed in the '855 patent yields base absorption maxima at ca. 335 nm, well into the UV-A region. Since the X.sub.1 substituents in the '855 patent are chosen to cause bathochromic (red shifts) or auxochromic (red shift and increased absorption strength), these materials are unable to effectively block UV-B wavelengths in contrast to the molecules of the present invention.
The compounds disclosed in the '937 patent suffer both from .beta.-hydrogen substitution, making them susceptible to nucleophilic conjugate addition during processing and from an inability to effectively block the UV-B region (methoxy substituted naphthyl chromophore).
The unique bis-.alpha.,.beta.-disubstituted cinnamic acid amides! (cinnamamides) of the invention are useful as stabilizers for engineering resins and other polymers which are processed at high temperatures (typically above 200.degree. C., most typically above 250.degree. C. and specifically from 275.degree.-325.degree. C.). Unlike the commercially available stabilizers, these cinnamamides have high UV absorption strength (including the UV-B range around 295 nm, a critically important portion of the action spectrum for engineered resins such as undyed polyesters and polycarbonates), low volatility, are thermally and chemically stable, do not undergo ester-ester interchange, and utilize a cis-trans photoisomerization as a light dissipation mechanism that does not involve an intramolecular proton transfer. The cinnamamides of the invention are sterically blocked via substitution in the .beta. position to prevent reactivity with nucleophilic end groups and additives, especially bases, found in the plastics being stabilized. The steric hindrance by functional groups attached to such a stabilizer double bond would also serve to prevent photodimerization.